Motion Control Keeps Electric Car’s Four Wheels on the Road
- Monday, 11 February 2013
It weighs half as much as a sports car, and turns on a dime—so its no surprise that the electric car being developed at Ohio State University needs an exceptional traction and motion control system to keep it on the road. With four wheels that turn independently, each with its own built-in electric motor and set of batteries, the experimental car is the only one of its kind outside of commercial carmakers’ laboratories.
In tests on good road conditions, the car followed a driver’s desired path within four inches (10 cm). To test slippery road conditions, the researchers took the car to an empty parking lot on a snowy day. There, the car maneuvered with an accuracy of up to eight inches (20 cm), and the vehicle traction and motion control system prevented “fishtailing” through independent control of the left and right sides of the car.
These results were characterized as more accurate than a conventional car, though the comparison is hard to make, given that conventional cars are much more limited in maneuverability by the transmission and differential systems that link the wheels together mechanically. The four independent wheels of the electric car give drivers greater control and more freedom of movement.
The experimental car also weighs half as much as a conventional car because it contains no engine, no transmission, and no differential. The researchers took a commercially available sport utility vehicle chassis and removed all those parts, and added a 7.5 kW electric motor to each wheel and a 15 kW lithium-ion battery pack. A single electrical cable connects the motors to a central computer.
One hundred times a second, the onboard computer samples input data from the steering wheel, gas pedal and brake and calculates how each wheel should respond. Because the wheels are independent, one or more can brake while the others accelerate, providing enhanced traction and motion control.
In fact, a driver who is accustomed to conventional cars would have a difficult time driving a car of this experimental design, known as a “four-wheel independently actuated” (FIWA) car without the help of the vehicle motion and traction control system. With its ability to turn sharply and change direction very quickly, the car could be hard to control. Wang has tried it.